1. Field of the Invention
The invention relates to a silicon-containing iron sheet for electrical applications. The invention also relates to methods for the manufacture of a silicon-containing iron sheet for electrical applications.
2. Description of the Prior Art
It is well known to alloy steel sheet with silicon for electrical applications, order to reduce power losses occurring with use of alternating current. These losses consist of two components, namely losses resulting from eddy currents and hysteresis losses. Eddy current losses reduce greatly as the content of silicon in the steel increases; hysteresis losses are dependent on impurities in the steel and irregularities in the crystal structure of the steel and increase slightly by alloying with silicon.
A frequent application of such steel sheet, to reduce power losses, is to be found in the form of flat or cylindrical sheet packs or stacks. Where the steel sheet thickness is small eddy current losses decrease greatly.
However, the present optimum in the silicon content and the thickness of such sheet is not solely determined by the requirements for reduction of power losses, but other factors also play their part. In the known steel sheet for electrical applications the final thickness is obtained by rolling, in other words it is a rolled product. With a silicon content exceeding 31/2% to 4% the steel becomes very difficult to cold-roll and can thus only be hot-rolled. At the same time the steel becomes brittle and consequently difficult to work, for example for subsequent die-stamping of laminates. Rolling costs are higher for small thicknesses so that the minimum practical thickness is also determined by economic factors.
In certain applications there are limits to the thickness of the steel sheet to be used, these limits relating to the stackability of the packs and their desired structural stiffness.
In practice, as a result of the above-mentioned circumstances, there is no industrial-scale manufacture of steel sheet for electrical applications with a silicon content exceeding 31/2% to 4% and with a thickness of under 0.15 mm.
Examples of processes described in the prior art of making silicon-containing steel sheet, known as silicon steel, are given in U.S. Pat. No. 3,423,253 and DE-A-2004272. U.S. Pat. No. 3,423,253 is concerned with increasing the silicon content of a wrought silicon steel strip, i.e. a product made by rolling, and describes deposition of silicon onto the silicon steel from vapour by thermal decomposition of a silicon compound, followed by heat-treating the steel to homogenize it. DE-A-2004272 is also concerned with increasing the silicon content of a silicon steel, made by a melting process, by deposition of silicon from vapour and heating to achieve a desired microstructure. JP-A-62-227035 has a similar disclosure. Such silicon steels contain other elements characteristic of steel-making processes, such as particularly C, Mn, P, S, etc.
GB-A-870870 and GB-A-1086215 on the other hand describe silicon-containing iron sheet, which is substantially pure Fe containing Si. To make this material, highly pure electrolytically deposited iron is used as a starting material for a melt for forming the Si-Fe alloy. The ingot cast from this melt is then rolled, to a thickness of 250 .mu.m or more.
It is mentioned for completeness that it is known to make thin sheet of pure iron by electrolytic deposition, as illustrated for example by EP-A-501548 and U.S. Pat. No. 4,076,597.